X-ray imaging systems have become invaluable in the medical field for a variety of surgical and diagnostic purposes. The implementation of many cardiac, urological, orthopedic, peripheral vascular, and a variety of non-invasive surgical procedures rely on the ability of the surgeon or medical authority to clearly track an implement they have inserted into a patient, such as a catheter, or otherwise monitor a region of interest within the patient through fluoroscopy. An example of a known fluoroscopy system is U.S. Pat. No. 2,730,566 issued to Bartow, et. al. entitled “Method and Apparatus for X-Ray Fluoroscopy. Computer Tomography (CT), in which a moving source-detector pair takes numerous two-dimensional images while rotating around a patient for reconstruction, is one of the preeminent methods of generating three-dimensional internal images used for cancer, other disease, and injury diagnoses. Single tomographic x-ray images are valuable for analysis as well.
The process of generating an x-ray image of a region of interest entails the positioning of a patient between an x-ray source and an x-ray detector, emission of x-rays from the x-ray source, the travel of these x-rays through a targeted volume of the patient, and the absorption of these x-rays by the x-ray detector. Since areas of a patient which are x-ray dense—notably, bones or vessels and tissues which have been highlighted by insertion of a contrast element—will absorb or scatter incident x-rays, the amount of x-ray photons reaching a given point on the x-ray detector corresponds to the x-ray density of the patient along a line between the x-ray source and that point on the detector. Therefore, intensity information from the detector can be used to reconstruct an image of the area of the patient through which the x-rays travelled.
Increasing the x-ray flux can improve image quality by increasing the amount of x-rays photons that pass through the patient and reach the detector, hence increasing the amount of intensity data available for image reconstruction. However, in addition to image quality considerations, decisions surrounding the x-ray flux are concerned with avoiding unnecessary exposure of the patient and attending medical personnel to x-ray radiation. While exposure of tissue to an extremely high amount of radiation at a given time would be necessary to see immediate negative health reactions such as radiation burns, a few relatively heavy doses to a patient or perpetual smaller doses to medical personnel may significantly increase probability of cancer later in life.
To maintain an x-ray flux sufficient for the generation of high-quality images while reducing x-ray exposure to system surroundings, an x-ray dense unit with a single aperture is generally positioned against the face of the x-ray source so that x-rays travelling along paths which, if uninterrupted, would not strike the detector face will be absorbed within its volume. The process of selectively attenuating x-rays is referred to as collimation, and the attenuating unit as a collimator.
Detector photon counts from absorption of scattered x-rays, which lower the image quality by contributing incorrect intensity information, are referred to as scatter noise. Systems have been developed with an “inverse geometry” such that the face of the x-ray source is relatively large and the face of the detector relatively small compared to conventional systems. Inverse geometry systems suffer significantly less from scatter noise as a smaller detector face decreases the probability of scattered ray absorption.
A notable type of inverse geometry systems is the scanning x-ray beam system such as the one disclosed in U.S. Pat. No. 5,729,584 entitled “Scanning Beam X-Ray Imaging System.” In scanning beam systems, x-ray beams are sequentially emitted from different points on the source, called focal spots, at very high speed rather than from the entire source face simultaneously. Since a number of images (corresponding to the number of emissive points on the source face) are used to reconstruct a single frame, the amount of patient volume exposed to x-rays at a given time, namely a narrow cone connecting a single aperture and the detector face, can be small compared to non-scanning systems where the entire target volume is continuously exposed. Scatter noise may be even lower in scanning beam systems as at a given time, scatter can only occur within this narrow illuminated cone rather than anywhere in the target volume. Information regarding the angular dependence of scanning beam images can also be used to add a three-dimensional, or tomographic, quality to the frames.
Non-conventional collimation devices are necessary for inverse geometry, scanning beam, and other multi focal spot x-ray imaging systems for a variety of reasons.
A multi focal spot collimator must direct x-rays from a source of large surface area to a small detector rather than from a small source to a large detector. This generally requires a plurality of closely-spaced apertures, each angled and shaped to emit x-rays that will intersect the detector face when illuminated by the source and attenuate x-rays that would spill around the detector face. Furthermore, in scanning beam systems, image reconstruction techniques rely on the assumption that x-rays are being emitted through only the intended aperture or intended apertures when a focal spot illuminates the collimator.
Additionally, while many single focal spot sources contain an x-ray reflective element so that the emissive portion of the source is positioned farther back in the body of the source, inverse geometry systems may require transmissive sources in which the target screen is the most outward element of the source. Material being constantly struck with high energy electrons and emitting Bremsstrahlung x-ray radiation will overheat without some sort of cooling system. Fast-moving, coolant fluid which absorbs and carries away excess heat is the key element in many cooling systems. Thus, in a system with a transmissive source, the collimator can be in contact with a coolant fluid system.
As a transmissive source may control the position of an electron beam with an applied magnetic field, any external electromagnetic fields may alter the beam path and disrupt the proper functioning of the x-ray source.
While the balance between x-ray image quality and dose control, improved by collimated multi focal spot systems, is particularly relevant in medical applications as discussed above, it can also be relevant in baggage screening, security applications, and other x-ray imaging applications.